Various embodiments of the concept of chemical leaching are known to those skilled in the art including, for example, vat leaching, pond leaching, pachuca tank leaching, and heap leaching. In recent years heap leaching has become commercially important, particularly for the extraction of gold and silver from their ores. See, for instance, P. G. Chamberlain and M. G. Pojar, Gold and Silver Leaching Practices in the United States, Bureau of Mines Information Circular No. 8969 (1984).
In the practice of heap leaching, heaps of ore, typically 20 to 30 feet high, are formed on impervious pads. Lixiviate containing water, heretofore sprinkled or sprayed upon the heap, percolates downward through the heap dissolving metal values, drains, passes through a recovery system, and is recycled to the sprinklers or sprayers. Heap leaching has the advantage of allowing large quantities of low grade ore to be leached at a relatively low cost. However, heap leaching as previously practiced has disadvantages. The lixiviate containing water spray is subject to evaporative loss and to windblown loss. Droplets of leach solution blown by the wind to the surroundings can contaminate the environment with toxic lixiviates such as sodium cyanide as well as with toxic substances leached from the ore. Spraying the lixiviate solution into the air leads to high rates of water evaporation. In addition to wasting water, evaporation causes heat to be lost thus lowering the temperature of the lixiviate solution and, consequently, lowering the chemical leaching reaction rate. Moreover, spraying the lixiviate solution into the air exposes a large surface area of the solution to sunlight thereby increasing potential photochemical degradation of photosensitive lixiviate compounds.